Chronic kidney disease (CKD) is characterized by the progressive loss of kidney function over a period of months or years. Recent professional guidelines classify the severity of CKD according to the reduction in glomerular filtration rate (GFR). Stage 1 CKD is the mildest disease condition which is accompanied by few symptoms and stage 5 CKD is the most severe disease condition which is characterized by poor life expectancy when remained untreated (NKF KDOQI GUIDELINES, 2002; Levey et al., Kidney Int. 67(6), 2089-2100, 2005; and Collins et al., Kidney Int. Suppl. 87, S24-S31, 2003). Untreated CKD may further progress into end stage disease wherein endogenous kidney function is irreversibly lost, a condition which renders the patient dependent upon dialysis or kidney transplantation. Approximately 90% of end stage renal disease is attributed to untreated CKD.
Early detection of kidney dysfunction is essential for the proper treatment of CKD and can prevent the progression of uremia which is a state of systemic poisoning due to the progressive loss of kidney function. In healthy population, “uremic” toxins are normally excreted by the kidneys. In CKD patients, the filtration of kidney nepgrons (assessed by the glomerular filtration rate (GFR)) diminishes thus causing an accumulation of “uremic” toxins, which are retained in the blood. Over 5000 potential toxins were found to be associated with the loss of kidney function including phenols, indoles, skatoles, pyridine derivatives, polyamines, aliphatic and aromatic amines, hippurate esters, hormones (e.g. prolactin), trace elements (e.g., aluminum, vanadium, arsenic, and zinc), guanidine compounds, serum proteases, and β2-microglobulin (Vanholder et al., J. Am. Soc. Nephrol. 19(5), 863-870, 2008; Jourde-Chiche et al., Semin. Dial. 22(4), 334-339, 2009; and Meyer et al., N. Engl. J. Med. 357(13), 1316-1325, 2007).
Currently used markers for loss of kidney function are retained urea and creatinine levels in urine and blood samples. However, serum creatinine levels vary widely with age, gender, diet, muscle mass, muscle metabolism, medications, and hydration status (Devarajan, Nephrology 15(4), 419-428, 2010). Moreover, up to 60% of kidney function may be lost before serum creatinine levels begin to rise. These limitations, and the asymptomatic onset of the disease, contribute to delayed diagnosis and treatment of CKD.
Di Natale et al. (Physiol. Meas. 20, 377-384, 1999) disclosed the use of an electronic nose to analyze the headspace of urine samples for detecting blood contents and measuring the pH and the specific weight of the samples. Fend et al. (Biosens. Bioelectron. 19(2), 1581-1590, 2004) disclosed the use of an electronic nose comprising an array of 14 conducting polymer sensors together with principal component analysis and hierarchical cluster analysis to analyze blood samples for monitoring dialysis.
A novel approach that overcomes many constraints of the conventional diagnostic techniques relies on patterns of volatile biomarkers in exhaled breath. Some of the volatile organic compounds (VOCs) from plasma CKD biomarkers or their metabolic products, are transmitted to the alveolar exhaled breath through exchange via the lung, even at the onset of the disease. At later stages, the characteristic VOCs are responsible for the fishy smell often detected in the breath of advanced CKD patients (Simenhoff et al., N. Engl. J. Med. 297(3), 132-135, 1977).
Moorhead et al. (Comput. Methods Programs Biomed. 101(2) 173-182, 2011) disclosed a two-compartment model for estimating glomerular filtration and further disclosed the use of selected ion flow tube-mass spectrometry (SIFT-MS) for monitoring of breath analytes to determine renal function.
Breath analysis for the diagnosis of uremia using an electronic nose device was demonstrated by Lin et al. (Sens. & Actuat. B, 76, 177-180, 2001). A sensor module composed of six 12 MHz AT-cut quartz crystals array coated with probe peptides designed by simulating the olfactory receptor protein docking with target gas molecules in combination with discriminating analysis, were used. Breath analysis of normal subjects, patients with uremia, patients with chronic renal insufficiency (CRI) and patients with chronic renal failure (CRF) provided discrimination with a correct classification of 86.78%.
Voss et al. (Ann. Biomed. Engin., 33(5), 656, 2005) disclosed the application of an electronic nose system based on doped semiconductor metal oxide gas sensors for studying human body odor in patients with different stages of renal insufficiency. Principle component analysis (PCA) followed by quadratic discriminant analysis produced discrimination of all healthy subjects from renal patients and further discrimination of dialysis patients from patients with chronic renal failure with a correct classification of 95.2% for two principle odor components and 98.4% for three principle odor components.
Haick and co-workers have recently demonstrated the ability of an array of carbon-nanotube-based sensors to differentiate between healthy states and induced end stage renal disease in rats via breath samples, using a model of bilateral nephrectomy, and have achieved a success rate of over 95% (Haick et al., ACS Nano 3(5), 1258-1266, 2009).
WO 2010/064239 to one of the inventors of the present invention discloses a system comprising an array of sensors of single-walled carbon nanotubes (SWCNTs) coated with functionalized oligomers or polymers for measuring VOCs as biomarkers for diagnosis, prognosis and monitoring of renal insufficiencies including, acute renal failure and chronic renal failure.
At present, no simple and reliable technique is available for early diagnosis of CKD and monitoring disease progression through breath analysis. There thus remains an unmet need for diagnosing early stage CKD in a non-invasive manner suitable for population screening in non-specialist settings.